From: Nikhil Engineer (ccienovice@gmail.com)
Date: Wed Apr 23 2008 - 04:56:17 ART
Noor,
R1 is accepting it as the best path must be due to shorter as-path.
but the main reason here is it is not adv it back to ibgp perr due to ibgp
split horizon rule.
Cheers,
Nikhil
On 4/23/08, Noor.Yousuf@shell.com <Noor.Yousuf@shell.com> wrote:
>
> Nikhil,
>
> I did some research and managed to find the right answer to this.
>
> R2 is advertising this prefix 200.200.200.0 to R1 and R4 through network
> statement. If you look at the BGP table of R1 you will see 200.200.200.0from both R2 and R3 but R2 will win the route because it has nothing in the
> AS-Path ( local to AS - shortest AS-Path). So, now because R1 has the best
> path through R2 it will not advertise the same prefix coming from R3(not the
> best route) to R2 -- Remember only best routes are advertise to peer.
>
> You have talked about LP, remember both IBGP and EBGP routes keeps the
> default LP of 100 at all time until you change it. the bgp table shows only
> the LP of ibgp route but when you do show ip bgp 200.200.200.0, you will
> see that both are carrying LP of 100. The decesion was made on shortest
> AS-Path.
>
> I hope this will clear any confusion
>
> Thanks again for the help
>
> Noor
>
> -----Original Message-----
> *From:* Nikhil Engineer [mailto:ccienovice@gmail.com]
> *Sent:* dinsdag 22 april 2008 19:04
> *To:* Yousuf, Noor S SITI-ITIBTAE
> *Cc:* Amir.Tahir@wateen.com; ccielab@groupstudy.com;
> itsfortarget@gmail.com; smorris@ipexpert.com; sadiqtanko@gmail.com;
> Patrick Galligan
> *Subject:* Re: BGP issue
>
> Hi Noor,
>
> I simulated your scenario here is the conclusion i came to:-
>
> R2 is originating the route so this route is preferred and advertised to
> R1.
> Now on R1 you will find two routes one is from R2 and other from R3. R2
> will be preferred cos of highest LP. As this is considered as the best route
> it wont be advertised to R2 due to "ibgp-split horizon rule"
>
> This is the only reason you will see that R2 has single route in bgp
> table.
>
> HTH.
>
> Cheers,
> Nikhil E.
>
> On Tue, Apr 22, 2008 at 3:34 PM, Patrick Galligan <pgalligan@gmail.com>
> wrote:
>
> > I've just done this on a couple of switches:
> >
> > 3750 - iBGP - 3560
> >
> > On both:
> > router bgp 200
> > net 10.0.0.0 mask 255.0.0.0
> > neigh blah
> >
> > ip route 10.0.0.0 255.0.0.0 null0
> >
> > I see both routes in the BGP table on both switches. I see the same
> > behaviour at customer sites where I'm originating their public block
> > from both routers to each ISP, with a static pointing to their
> > firewall/s, and run iBGP between the customer routers.
> >
> > Noor, have run done any debugs?
> >
> > eg. deb ip bgp update
> >
> > and then
> >
> > clear ip bgp <neighbour address>
> >
> > Are you running on Dynamips or real routers? What IOS?
> >
> > On Tue, Apr 22, 2008 at 5:47 PM, Nikhil Engineer <ccienovice@gmail.com>
> > wrote:
> > > hi Noor,
> > >
> > > As I said before this is due to split horizon.
> > >
> > >
> > > Cheers,
> > > Nikhil E
> > >
> > >
> > > On 4/22/08, Noor.Yousuf@shell.com <Noor.Yousuf@shell.com> wrote:
> > > >
> > >
> > >
> > > > Amir,
> > > >
> > > > I understand your point, and I know that it is locally originated
> > again
> > > > and it is in the bgp table but I would only like to know the reason
> > why the
> > > > same route from R3 is not in the BGP table anymore?
> > > >
> > > > Thanks,
> > > >
> > > > -----Original Message-----
> > > > From: Amir.Tahir/Wateen/Lahore [mailto:Amir.Tahir@wateen.com]
> > > > Sent: dinsdag 22 april 2008 9:28
> > > > To: Yousuf, Noor S SITI-ITIBTAE; ccielab@groupstudy.com
> > > > Cc: itsfortarget@gmail.com; smorris@ipexpert.com;
> > sadiqtanko@gmail.com
> > > > Subject: RE: BGP issue
> > > >
> > > >
> > > > Dear Noor,
> > > >
> > > > When you have advertised 200.200.200.0/24 on R3, why you are
> > advertising
> > > > same network Prefix on R2.
> > > >
> > > > If you have eBGP relationship between R1 & R3, you should learn 200
> > > > prefix from R3 and should be visible on R2 as well. You don't have
> > to
> > > > re-advertise it.
> > > >
> > > > If you have eBGP relation between R2 & R4, it should be propagated
> > to R4
> > > > through the AS (Defined on R2 & R1)...
> > > > If Ip route command confuse you, try to run rip between R1 & R2 to
> > have
> > > > the NLRI reachibility..
> > > >
> > > > As far as you query is concern... you have defined the 200 prefix
> > > > through Network command on R2 and now its locally advertised. You
> > could
> > > > verify it through Weight (32768) & AS Path (i).
> > > >
> > > >
> > > > r2#SH IP BGP
> > > > BGP table version is 8, local router ID is 2.2.2.2
> > > > Status codes: s suppressed, d damped, h history, * valid, > best, i
> > -
> > > > internal,
> > > > r RIB-failure, S Stale
> > > > Origin codes: i - IGP, e - EGP, ? - incomplete
> > > >
> > > > Network Next Hop Metric LocPrf Weight Path
> > > > *> 200.200.200.0 0.0.0.0 0 32768 i
> > > >
> > > >
> > > > If still you are not clear, Please feel free to call me at
> > > > 0322-400-2647(amir)
> > > >
> > > >
> > > > Regards
> > > > __________________
> > > > Amir Tahir
> > > >
> > > >
> > > > -----Original Message-----
> > > > From: Noor.Yousuf@shell.com [mailto:Noor.Yousuf@shell.com]
> > > > Sent: Tuesday, April 22, 2008 12:12 PM
> > > > To: Amir.Tahir/Wateen/Lahore; ccielab@groupstudy.com
> > > > Cc: itsfortarget@gmail.com; smorris@ipexpert.com;
> > sadiqtanko@gmail.com
> > > > Subject: RE: BGP issue
> > > >
> > > > Let me get clear..
> > > >
> > > > R3----EBGP---R1----IBGP-----R2----EBGP----R4
> > > >
> > > >
> > > > from R3 I have advertised the network 200.200.200.0 in BGP
> > > > no sync
> > > > network 200.200.200.0 mask 255.255.255.0
> > > > ip route 200.200.200.0 255.255.255.0 Null0
> > > >
> > > > I can see the network on R2 bgp table
> > > > Now I have advertised the same network in R2
> > > > no sync
> > > > network 200.200.200.0 mask 255.255.255.0
> > > > ip route 200.200.200.0 255.255.255.0 Null0
> > > >
> > > > As soon as I have advertised the network in R2 BGP, I cannot see
> > the
> > > > route coming from R1 in the BGP Table, I thought I should see both
> > > > routes in BGP table but only one in Routing table.
> > > >
> > > > r2#SH IP BGP
> > > > BGP table version is 8, local router ID is 2.2.2.2
> > > > Status codes: s suppressed, d damped, h history, * valid, > best, i
> > -
> > > > internal,
> > > > r RIB-failure, S Stale
> > > > Origin codes: i - IGP, e - EGP, ? - incomplete
> > > >
> > > > Network Next Hop Metric LocPrf Weight Path
> > > > *> 200.200.200.0 0.0.0.0 0 32768 i
> > > >
> > > > Can someone explains why?
> > > >
> > > > Thanks,
> > > >
> > > >
> > > > Pass the CCIE in six weeks, Guaranteed!
> > > > http://www.certscience.com/CCIE
> > > >
> > _______________________________________________________________________
> > > > Subscription information may be found at:
> > > > http://www.groupstudy.com/list/CCIELab.html
> > >
> > >
> > > Pass the CCIE in six weeks, Guaranteed!
> > > http://www.certscience.com/CCIE
> > >
> > _______________________________________________________________________
> > > Subscription information may be found at:
> > > http://www.groupstudy.com/list/CCIELab.html
> > >
> > >
> > >
> > >
> >
> >
> >
> >
> > --
> > Certain techniques work on a motorcycle. Other techniques don't, and
> > they hurt
> >
> >
> > Pass the CCIE in six weeks, Guaranteed!
> > http://www.certscience.com/CCIE
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
Pass the CCIE in six weeks, Guaranteed!
http://www.certscience.com/CCIE
This archive was generated by hypermail 2.1.4 : Thu May 01 2008 - 08:25:51 ART